Method for manufacturing mechanical pulp and use of the mechanical pulp

ABSTRACT

A method for treating mechanical pulp. Resinous mechanical pulp is brought into process in a process water flow. The resinous pulp is ozonated at a temperature between 80 and 87° C. and at pH less than 4. The pH of the pulp is adjusted after the ozonation process to correspond to the pH required in bleaching. The pulp is bleached.

The present invention relates to a method for treating mechanical pulpand a use of the mechanical pulp.

Pitch is a harmful substance in mechanical pulp manufacture. When theraw material is spruce the pitch content is quite low so that it is notprobable that the pitch will cause any uncontrolled problems. However,if the raw material is pine, which contains more pitch than spruce(about three times more), the pitch or pitch and fibers will deposit onsurfaces during the processing of the resinous raw material, in a papermachine, or a printing machine, and thereafter detrimental effects areto be expected. Thus, in order to use pine as a pulpwood, the pitch mustbe reduced or removed, or it must be modified to less harmfulcomponents.

The above-mentioned tasks can be accomplished by the method of theinvention whose characteristic features are conformable to claim 1.

The pulp is treated with ozone in order to remove, reduce or modify thepitch. The pitch is one of the first pulp components with which ozonereacts. However, ozone also tend to react with lignin and as a result,chromophoric compounds are released into the process water. Therefore,the brightness of the pulp is deteriorated, and it cannot fully berecovered afterwards in a subsequent bleaching step in which eitherdithionite or peroxide is used.

By the method of the invention, it is possible that the pitch isreduced, removed or modified by treating the pulp with ozone so that theoptical properties can be recovered by the subsequent bleaching step. Inpractice, it may be adequate that the pitch content of pine is reducedso that its pitch content after the ozonation corresponds to the pitchcontent of spruce.

In the method of the invention, the starting raw material consists oflogs which have been ground in a PGW (pressure groundwood) process or aGW (groundwood) process. The logs are softwood species, preferably pine(Pinus sylvestris), or Southern pine (genus Pinus, several differentspecies).

Pine is a highly resinous material whose utilization in mechanicalpulping has been a problem because the resinous part of the material isharmful in a process. Pine may be used as a mixture with spruce ingroundwood (GW) or pressure groundwood (PGW) pulp. For example, the pinecontent of the pulp may be 20 to 60 wt.-%, or the pulp may entirelyconsist of pine. In general, the present method is useful when the pitchcontent of the wooden raw material is more than 0.8 wt.-% and moreuseful when the pitch content is more than 2.0 wt.-%. However, nothingprevents the use of the method for raw materials whose pitch content is0.8 wt.-% or lower but the benefits may be smaller with those materials.

In the method of the invention, resinous mechanical pulp is brought intoprocess in a process water flow. The pulp is ozonated at a consistencywhich is between 0.5 and 4%. The pulp is ozonated so that ozone isinjected to a mass flow before a container, such as a mass pulp silo.The resinous pulp is ozonated at a temperature which is above 70° C.,usually between 80 and 87° C., preferably between 80 and 85° C. In orderto achieve the above-mentioned temperature ranges, it is normallyrequired to lower the temperature of the circulating water because thenormal temperature is around 95° C.

The ozonation process produces acidic by-products, such as organicacids, which reduce the pH. Therefore, the ozonation takes place at pHwhich is less than 4 if no means for increasing the pH has been used.However, there is no need to increase the pH because it has beendiscovered that if the pH is increased over 4 it causes darkening of thepulp. Thus, the natural drop of the pH leads to favourable conditionsfor treating the pulp if the dose of the ozone is kept on a level lessthan 1 g/1 g pitch to be split. The pulp is preferably ozonated in morethan one step; for example, two subsequent steps may produce betterresult than only one step. In practice, there may be two subsequentsilos before which there are injectors which bring the ozone in theprocess. Before the first silo the pH may be 3.4 and before the secondsilo 3.2, the pH may preferably vary between 3.2 and 3.4. The ozonereacts almost immediately with the pitch so that in the gas exiting fromthe process there is not significant amounts of residual ozone, or thereis not at all ozone.

The total amount of ozone in several steps in series is equal to or lessthan 1 g ozone/1 g pitch to be split, preferably 0.6-0.8 g O₃/1 g pitch.The amount of the pitch is a theoretical amount which has beencalculated by using estimations that spruce contains 0.8 wt.-% of pitchand pine contains 2.4 wt.-% of pitch, one ton of spruce theoreticallycontains 8000 g pitch, and one ton of pine theoretically contains 24000g pitch. The practical target is to reduce the pitch content of pine sothat it is on the same level with the pitch content in spruce. Normally,the pitch of spruce does not affect pulp or papermaking processes. Anoverdose of ozone should be avoided because ozone, as mentioned above,tends to react with lignin and as a result, the process water and thepulp will darken.

It is possible to decrease the temperature of the circulating water ofthe groundwood mill by leading replacement water of lower temperature tothe circulating water. The replacement water may be circulating waterfrom a paper machine which may be situated in the flow direction behindthe groundwood mill. Besides lowering the temperature, the replacementwater is cleaner than the circulating water of the groundwood mill andthus, the relative amount of the chromophoric compounds reduces in thecirculating water and the darkening of the pulp through the processwater is reduced. The replacement water may be led in a filtrate chestin which it mixes with the circulating water. Excess circulating wateris led out of the process. Heat is recovered from the water by heatexchangers.

After the ozonation the pH of the pulp is adjusted to correspond to thepH of a bleaching step. Thereafter the pulp is bleached by using, forexample, dithionite or peroxide. A typical pH range for dithionitebleaching for ozonated pulp is from 4.7 to 5.3, and for peroxidebleaching around 10.5. Therefore, the level to which the pH is adjustedbefore bleaching is dependent on the subsequent bleaching process.Suitable pH adjusting agents include sodium hydroxide (NaOH), sodiumbicarbonate (Na₂CO₃), magnesium hydroxide (Mg(OH)₂), calcium hydroxide(Ca(OH)₂), or their mixtures.

According to one embodiment of the invention, the above-mentionedpreferred temperature range, the range from 80 to 85° C., may be widerand/or higher ( temperatures up to 95° C. are possible) but replacementwater is required to clean the circulating water.

The obtained pulp will be used in a paper manufacture process forproducing mechanical pulp containing paper grades, such as machinefinished specialty (MFS) papers for advertising material, directmailing, magazines, newspaper inserts and newspapers, newsprint,supercalendered, or base papers for light weight coated papers. Thepapermaking fibers of such grades may entirely consist of mechanicalpulp fibers. It is possible that the mechanical pulp in the paperentirely consists of the ozonated pine but it is also possible that onlya certain amount of the mechanical pulp is pine and has been obtainedfrom the process in question. For example, the ozonated GW or PGW pinemay be used as a mixture with thermomechanical (TMP) spruce, or chemicalpulp. Typical composition of furnishes of the above-mentioned papersgrades may be:

PGW TMP Chemical pulp Filler Paper grade (wt.-%) (wt.-%) (wt.-%) (wt.-%)Newsprint 40-50 50-60 0 0-10 SC 50 — 25 25 LWC, base 55 — 35 10 paper

EXAMPLE

Eight industrial scale trials were made. Estimations that sprucecontains 0.8 wt.-% of pitch and pine contains 2.4 wt.-% of pitch wereused, and the required amount of ozone was calculated by using theestimations. The target was that the pitch content of the ozonated pulpshould not exceed 0.8 wt.-%. The results of the calculations are shownin table 1.

TABLE 1 Calculations for industrial scale trials. PGW pine Average pitchPitch to be Pitch to be Amount of proportion in a pulp removed removedozone Trial (wt.-%) (wt.-%) (wt.-%) (kg/ton) (kg/pitch kg) 1 50 1.600.80 8.00 1.29 2 20 1.12 0.32 3.20 0.81 3 20 1.12 0.32 3.20 0.40 4 331.33 0.53 5.28 0.75 5 33 1.33 0.53 5.28 0.70 6 33 1.33 0.53 5.28 1.49 750 1.60 0.80 8.00 1.03 8 50 1.60 0.80 8.00 0.82

It was found that the amount of pitch was on an acceptable level afterthe ozonation. The pulps were led to the paper manufacture. Besides thetrial pulp, each paper contained from 5 to 10 wt.-% inorganic filler.The obtained papers did not leave any pitch deposits on roll surfaces.Good results were obtained in printing trials.

Next, the invention will be described referring to a drawing which showsa layout of the ozone treatment process.

FIG. 1 shows a layout of the ozone treatment process. Mechanical pulphas been ground in a grinder from logs, preferably pine logs, tomechanical pulp. There may be several parallel grinders. The mechanicalpulp slurry is led to a buffer tank 1 from which it is pumped by pumps 2to a mass transfer piping 3.

The pulp slurry is led via the mass transfer piping 3 to a booster pump4 which raises the pressure of the slurry. An injector 5 bringsozone/oxygen mixture to the slurry from a ozone generating system 8. Theozone generating system 8 (marked by a dashed line) comprises an ozonegenerator 9, pumps 10 and heat exchangers 11. Mixers 6 stir theozone/oxygen mixture into the slurry. As can be seen from FIG. 1, theremay be parallel pumps 4, injectors 5 and mixers 6. The ozonated slurryis led to a first mass pulp silo 7. In the first mass pulp silo, theozone reacts with the pitch in the pulp in a first ozonating step sothat the pitch is reduced, removed or modified.

After the first ozonating step, principally the same process step isrepeated in a second ozonating step. An injector 13 brings ozone/oxygenmixture to the slurry from the ozone generating system 8. Mixers 14 stirthe ozone/oxygen mixture into the slurry. There may be parallel pumps12, injectors 13 and mixers 14. The ozonated slurry is led to a secondmass pulp silo 15. In the second mass pulp silo 15, the ozone reactswith the pitch in the pulp in a second ozonating step so that the pitch,which is left after the first ozonating step, is reduced, removed ormodified.

After the pulp has been treated with the ozone, the ozone is led to anozone destruction section 16 (marked by a dashed line). Ozone isconverted to oxygen, and released from the process.

The pulp is led from the second mass pulp silo 15 to a bleaching step(not shown). Before the bleaching, the pulp is treated with alkali sothat its pH is suitable for the following step. Suitable alkalis may besodium hydroxide (NaOH), sodium bicarbonate (Na₂CO₃), magnesiumhydroxide (Mg(OH)₂), calcium hydroxide (Ca(OH)₂), or their mixtures.

One skilled in the art understands readily that the layout of theprocess may be different from the process illustrated in FIG. 1 but itstill works according to the basic principle described in thisapplication.

1. A method for treating mechanical pulp, the method comprising:bringing resinous mechanical pulp into a process in a process waterflow, ozonating the resinous pulp at a temperature which is between 80and 87° C., and at pH which is less than 4, adjusting the pH of the pulpafter the ozonation process to correspond to a pH required in bleaching,and bleaching the pulp.
 2. The method according to claim 1, wherein theresinous pulp is ozonated in more than one step.
 3. The method accordingto claim 1, wherein an amount of ozone is at most 1 g/1 g pitch to besplit calculated from a theoretical pitch content.
 4. The methodaccording to claim 2, wherein the resinous pulp is ozonated before twosuccessive mass pulp silos.
 5. The method according to claim 1, whereina part of the process water flow is replaced by water which is cleanerthan the process water flow after ozonating the pulp but beforebleaching the pulp.
 6. The method according to claim 5, wherein the partof the process water flow is replaced by circulating water from a papermachine.
 7. The method according to claim 1, wherein the pulp isozonated at a consistency which is between 0.5 and 4%.
 8. The methodaccording to claim 1, wherein the pulp is bleached with dithionite orperoxide.
 9. The method according to claim 1, further comprising:utilizing the pulp in paper manufacture.
 10. The method according toclaim 8, wherein the pulp is at least partially made of pine.
 11. Themethod according to claim 8, wherein the pulp consists of pine.
 12. Themethod according to claim 1, wherein the resinous pulp is ozonated at atemperature which is between 80 and 85° C.
 13. The method according toclaim 1, wherein the resinous pulp is ozonated in two steps.